Low ni-containing steel alloys with hydrogen degradation resistance

ABSTRACT

The present invention provides steel alloys with hydrogen degradation resistance comprising controlled amounts of Mn and C, as well as Al, Cr, Cu, Ni and Si. The steel alloys have an austenite microstructure and relatively high stacking fault energies, which avoid the formation of martensitic phases that reduce hydrogen resistance.

CROSS-REFERENCE TO RELATED APPLICATION

This application claims the benefit of U.S. Provisional PatentApplication No. 63/336,431 filed Apr. 29, 2022, which is incorporatedherein by reference.

FIELD OF THE INVENTION

The present invention relates to low-Ni steel alloys with favorableresistance to hydrogen degradation during service.

BACKGROUND INFORMATION

Alloys currently used for high pressure hydrogen storage applicationsinclude grade 316L austenitic stainless steel that contains nominally 18weight percent Cr and 13 weight percent Ni in addition to iron andseveral other elements. However, Cr and Ni additions are relativelyexpensive, and a lower cost alternative would benefit hydrogenapplications such as the use of hydrogen as a fuel for automobiles,trucks and the like.

SUMMARY OF THE INVENTION

The present invention provides steel alloys with hydrogen degradationresistance comprising controlled amounts of Mn and C, as well as Al, Cr,Cu, Ni and Si. The steel alloys have an austenite microstructure andrelatively high stacking fault energies, which avoid the formation ofmartensitic phases that reduce hydrogen resistance.

An aspect of the present invention is to provide a hydrogen degradationresistant steel alloy comprising from 15 to 30 weight percent Mn, from0.15 to 1 weight percent C, and from 0.05 to 3 weight percent Al. Thesteel alloy has a microstructure comprising at least 99 percent volumeaustenite, and possesses a relative reduction in area of no more than 20percent.

This and other aspects of the present invention will be more apparentfrom the following description.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a graph illustrating that fully austenitic steels withdesirable stacking fault energies may be achieved using relatively largeamounts of Mn with additions of controlled amounts of C, Al and Ni inaccordance with the present invention.

FIG. 2 is a graph illustrating the achievement of suitable stackingfault energies based upon Mn content with further additions of Al, Cr,Cu and Ni in accordance with embodiments of the present invention.

FIGS. 3-6 are graphs of mechanical properties and hydrogen content forhydrogen degradation resistant steel alloys of the present invention incomparison with a standard stainless steel alloy. FIG. 3 illustratesultimate tensile strengths, FIG. 4 illustrates yield strengths, FIG. 5illustrates total elongations, and FIG. 6 illustrates relativereductions of area.

FIGS. 7 and 8 are three-dimensional plots for steel alloys of differentcompositions having varying amounts of Mn and C, and the resultanteffect on relative reduction of area for the steel alloys.

FIGS. 9-14 are photomicrographs of fracture surfaces of a hydrogendegradation resistant steel alloy of the present invention before andafter electrochemically hydrogen charging taken at different locationsof the sample, illustrating ductile fracture features.

DETAILED DESCRIPTION

The present steel alloys may be used for hydrogen service due to theirrelatively high stacking fault energies, e.g., greater than 20 mJ/m², toavoid formation of martensitic phases which greatly reduce hydrogenresistance. Alloy compositions that stabilize austentite and avoidmartensite formation may be selected in accordance with the presentinvention.

The hydrogen-resistant steel alloys of the present invention maytypically comprise at least 15 weight percent Mn, for example, at least18 weight percent, or at least 20 weight percent, or at least 20.5weight percent, or at least 21 weight percent, or at least 22 weightpercent. The Mn may comprise up to 30 weight percent, or up to 25 weightpercent, or up to 24 weight percent. In certain embodiments, the Mn maycomprise from 15 to 30 weight percent, or from 18 to 25 weight percent,or from 20 to 24 weight percent. The relatively large amount of Mn mayprovide similar qualities as stainless steel, including resistance tohydrogen degradation during service. The Mn content leads to fullyaustenitic steels that resist degradation effects of hydrogen such asductility loss or embrittlement.

The hydrogen-resistant steel alloys may typically comprise at least 0.18weight percent C, for example, at least 0.25 weight percent, at least0.3 weight percent, or at least 0.4 weight percent. The C may compriseup to 1 weight percent, or up to 0.9 weight percent, or up to 0.8 weightpercent, or up to 0.6 weight percent. In certain embodiments, the C maycomprise from 0.18 to 1 weight percent, or from 0.25 to 0.9 weightpercent, or from 0.3 to 0.8 weight percent, or from 0.4 to 0.6 weightpercent.

The hydrogen-resistant steel alloys may typically comprise at least 0.05weight percent Al, for example, at least 0.1 weight percent, or at least0.5 weight percent, or at least 0.8 weight percent, or at least 1.0weight percent, or at least 1.2 weight percent. The Al may comprise upto 2.5 weight percent, or up to 2.2 weight percent, or up to 2 weightpercent, or up to 1.8 weight percent. In certain embodiments, the Al maycomprise from 0.05 to 2.5 weight percent, or from 0.8 to 2.2 weightpercent, or from 1 to 2 weight percent, or from 1.4 to 1.8 weightpercent.

The hydrogen-resistant steel alloys may typically comprise at least 0.5weight percent Si, for example, at least 1 weight percent, or at least 2weight percent, or at least 2.5 weight percent. The Si may comprise upto 4 weight percent, or up to 3.5 weight percent, or up to 3.2 weightpercent, or up to 3 weight percent. In certain embodiments, the Si maycomprise from 1 to 4 weight percent, or from 1.5 to 3.5 weight percent,or from 2 to 3.2 weight percent, or from 2.5 to 3 weight percent. Incertain embodiments, the steel alloys may be substantially free of Si.

The hydrogen-resistant steel alloys may typically comprise at least 0.8weight percent Ni, for example, at least 1 weight percent, or at least1.2 weight percent. The Ni may comprise up to 2.5 weight percent, or upto 2 weight percent, or up to 1.5 weight percent. In certainembodiments, the Ni may comprise from 0.8 to 2.5 weight percent, or from1 to 2 weight percent, or from 1.2 to 1.5 weight percent. In certainembodiments, the steel alloys may be substantially free of Ni.

The hydrogen-resistant steel alloys may typically comprise at least 0.2weight percent Cu, for example, at least 0.4 weight percent Cu, or atleast 0.6 weight percent Cu. The Cu may comprise up to 2 weight percent,or up to 1.5 weight percent, or up to 1.2 weight percent. In certainembodiments, the Cu may comprise from 0.2 to 2 weight percent, or from0.4 to 1.5 weight percent, or from 0.6 to 1.2 weight percent. In certainembodiments, the steel alloys may be substantially free of Cu.

The hydrogen-resistant steel alloys may typically comprise at least 1weight percent Cr, for example, at least 1.5 weight percent Cr, at least2 weight percent Cr, or at least 2.2 weight percent Cr. The Cr maycomprise up to 3.5 weight percent, or up to 3.2 weight percent, or up to3 weight percent, or up to 2.8 weight percent. In certain embodiments,the Cr may comprise from 1.5 to 3.5 weight percent, or from 2 to 3.2weight percent, or from 2 to 3 weight percent, or from 2.2 to 2.8 weightpercent. Alternatively, the Cr may be less than 1.5 weight percent, orless than 1 weight percent, or less than 0.5 weight percent, or lessthan 0.2 weight percent. In certain embodiments, the steel alloys may besubstantially free of Cr.

The hydrogen-resistant steel alloys may typically comprise at least 0.01weight percent Ti, for example, at least 0.05 weight percent, or atleast 0.08 weight percent. The Ti may comprise up to 0.5 weight percent,or up to 0.3 weight percent, or up to 0.2 weight percent. In certainembodiments, the Ti may comprise from 0.01 to 0.5 weight percent, orfrom 0.02 to 0.3 weight percent, or from 0.08 to 0.2 weight percent. Incertain embodiments, the steel alloys may be substantially free of Ti.

As used herein, the term “substantially free” when referring to alloyingadditions, means that a particular element or material is notpurposefully added to the alloy, and is only present, if at all, inminor amounts as an impurity. For example, in amounts of less than 0.05weight percent, or less than 0.01 weight percent.

The hydrogen degradation resistant steel alloys have an austeniticmicrostructure in which austenite comprises at least 95 volume percent,or at least 98 volume percent, or at least 99 volume percent, or atleast 99.5 volume percent. Other than austenite, the hydrogendegradation resistant steel alloys may be substantially free of otherphases such as ferrite and martensite. For example, such phases, ifpresent, are less than 1 volume percent, or less than 0.5 volumepercent, or less than 0.1 volume percent, or zero volume percent.

FIGS. 1 and 2 illustrate the design concept for the present invention.FIG. 1 indicates that fully austenitic steels with the target SFE rangemay be achieved using relatively high amounts of Mn, e.g., 22 weightpercent and 15 weight percent, with additions of suitable amounts of C,e.g., 0.45 weight percent C plus Al, Cu and Ni. An alloy with only 0.18weight percent C and 15 weight percent Mn may not meet the design goalfor SFE. FIG. 2 further shows that for the carbon and manganese contentsstudied, SFE falls into the desired range with further additions of Al,Cr, Cu and Ni.

Laboratory scale heats of each composition listed in Table 1 are melted,hot rolled and prepared for electrochemical charging to form nascenthydrogen at the sample surface. The electrochemical charging techniquewas performed by electrochemically charging the test samples in asolution of 20 g/L Na₂SO₄ for 48 hours at 70 C with additions of 2 g/LNH₄SCN to prevent recombination of the nascent hydrogen. A currentdensity of 70 A/m2 was used for the test. During the electrochemicalcharging, the nascent atomic hydrogen diffuses into the test samples.

Hydrogen charged samples may be tested for hydrogen resistance byperforming standard tensile tests and comparing ductility with samplesthat are not charged. Reduction in Area (RA) may be used to measureductility. A target for Relative Reduction in Area (RRA) of 20% isconsidered to be competitive with 316L stainless steel. Thus, if RAdegrades no more than 20% then alloys of the present invention areconsidered to be competitive with 316L stainless from a hydrogenresistance standpoint.

The following examples are intended to illustrate various aspects of thepresent invention, and are not intended to limit the scope of theinvention.

Examples

Laboratory melts were made in a vacuum induction furnace with the actualchemistries shown in Table 1.

TABLE 1 Melt Chemistries Alloy Compositions (values in weight percent)Al loy C Mn Al Cr Cu Ni Si Ti 2 Aim 0.18 22 1.5 2.5 0.8 1.3 2.7 — 2Actual 0.18 20.5 1.43 3.2 0.645 0.994 3.20 3 Aim 0.45 22 1.5 2.5 0.8 1.32.7 — 3 Actual 0.478 24.0 1.36 2.44 0.811 1.33 2.60 4 Aim 0.45 22 1.52.5 0.8 1.3 2.7 0.1 4 Actual 0.48 24.0 1.38 2.44 0.834 1.35 2.61 0.09885 Aim 0.18 15 1.5 2.5 0.8 1.3 2.7 — 5 Actual 0.198 16.5 1.44 2.49 0.8281.37 2.70 6 Aim 0.45 15 1.5 2.5 0.8 1.3 2.7 0.1 6 Actual 0.478 16.7 1.452.51 0.859 1.39 2.65 0.122

The chemistries were measured either by a LECO C/N/O/S Analyzer or byInductively Couple Plasma Optical Emission Spectroscopy (ICP-OES).Titanium was added to some of the melts for microalloying to improveyield strength, in addition to possibly reducing the kinetics of twinformation. Low levels of phosphorus, 0.015 weight percent, and sulfur,0.005 weight percent, were added to each alloy to simulate residualphosphorus and sulfur in a steel melt. The material was hot rolled froma 7-inch-thick ingot to a 1.25-inch-thick slab in the laboratory and aircooled. All testing was completed on the hot rolled slabs.

The samples were measured with a Metis MSAT 30 instrument to determinethe percentage of austenite in the material. The results are listed inTable 2 and compared against the 316L stainless steel material used inthis study. A fully austenitic structure is desirable to preventhydrogen embrittlement. Since the samples will be stored in liquidnitrogen to prevent de-absorption of the hydrogen, the samples were alsotested after a 24-hour storage in liquid nitrogen. There were noindications of microstructural changes after storage in liquid nitrogen,except the percent of austenite in Alloys 4 and 6 reduced by 0.1 volumepercent. The alloys were almost fully austenitic microstructures.

TABLE 2 Percent of Austenite in the Microstructure Alloy % Austenite %Austenite After Liquid Nitrogen 316L SS 99.6 99.6 2 99.7 99.7 3 99.999.9 4 99.9 99.8 5 99.8 99.8 6 99.9 99.8

ASTM E8-22 Specimen 2 round tensile samples in the longitudinaldirection, parallel to the rolling direction, were tested according tothe ASTM E8-22 standard before and after electrochemically charging forhydrogen. The extensometer range was exceeded during testing, so thetotal elongation was manually measured for all the samples. The tensilesamples along with a hydrogen analysis test sample wereelectrochemically charged in a solution of 20 g/L Na₂SO₄ for 48 hours at70 C with a current density of 70 A/m2. Additions of 2 g/L NH₄SCN wasadded to prevent recombination of the nascent hydrogen. The hydrogentest samples were selected from the same melt and near the same locationas the tensile sample to minimize parameters that could affectquantities of hydrogen adsorption, such as grain size. Immediately afterelectrochemically charging for hydrogen, the tensile samples were storedin liquid nitrogen to await tensile testing. There was an 8 to 12 minutedelay for the tensile sample temperature to stabilize to roomtemperature prior to testing. The mechanical properties before and afterelectrochemically charging were compared, along with the concentrationof diffusible hydrogen from the hydrogen test sample that was measuredwith a Bruker hydrogen analyzer mass spectrometer at 300 C.

The mechanical property results before and after electrochemicallycharging, along with the hydrogen concentrations are shown in Table 4and FIGS. 3-6 , compared against 316L stainless steel material. It issuspected that the higher hydrogen concentration in 316L stainless steelmay be due to microstructural differences, such as grain size and thepercentage of ferrite, compared to the hot rolled alloys in this study.The mechanical properties were all comparable or higher in strength andtotal elongation than the 316L stainless steel material. Alloys 2, 3, 4and 6 have relative reduction of areas (RRA) less than 20% with Alloy 3performing the best. The RRA of Alloy 5, which had both lower carbon andmanganese levels of 0.2 weight percent and 16.5 weight percent,respectively, was 23.2%. Increasing the manganese levels to 20.5 weightpercent as in Alloy 2 improves the average RRA to a value of 11.8% andincreasing the carbon levels to 0.48 weight percent as in Alloy 6improves the RRA to an average value of 14.1%. Increasing both thecarbon and the manganese levels to 0.48 weight percent carbon and 24.0weight percent manganese further improves the RRA to an average value of2.6% for Alloy 3 and 11.3% for Alloy 4.

FIGS. 7 and 8 visually show the effects on Mn and C contributions on theRRA. The data in FIGS. 7 and 8 show a decrease in RRA as the C and Mnconcentrations increase, and that both Mn and C contribute to the RRAindividually. The addition of titanium in Alloy 4, compared againstAlloy 3, increased the tensile and yield strength but also increased theRRA value.

TABLE 3 Mechanical properties before and after hydrogen charging andmeasured hydrogen concentrations of test samples Before After BeforeBefore Charging After After Charging Charging Charging Avg TE % BeforeCharging Charging TE % After Hydrogen Avg 0.2% Avg UTS (manuallyCharging 0.2% OYS UTS (manually Charging RRA 300 C Melt OYS (MPa) (MPa)measured) Avg RA % (MPa) (MPa) measured) RA % (%) (ppm) 316L SS 386 64848.8 74.4 322.0 623.0 54.0 70.0 5.9 5.04 Alloy 2 370 765.5 59.4 73.6352.0 745.0 59.0 64.4 12.5 1.08 Alloy 2 370 765.5 59.4 73.6 345.0 724.059.0 65.4 11.2 0.79 Alloy 3 400 848 67.85 63.16 386.0 807.0 65.9 61.23.1 1.82 Alloy 3 400 848 67.85 63.16 379.2 806.7 63.4 62.2 1.5 1.38Alloy 3 400 848 67.85 63.16 406.8 820.5 64.0 60.7 3.9 1.51 Alloy 3 400848 67.85 63.16 393.0 813.6 66.1 62.1 1.7 1.48 Alloy 4 437.5 886 61.4559.79 427.5 861.8 59.9 53.7 10.2 1.05 Alloy 4 437.5 886 61.45 59.79448.2 882.5 58.5 53.6 10.4 1.91 Alloy 4 437.5 886 61.45 59.79 420.6848.1 59.0 51.8 13.4 1.43 Alloy 5 358.5 823.5 57.3 61.18 344.7 779.153.9 47.0 23.2 0.93 Alloy 6 444.5 899.5 61.45 62.74 441.3 882.5 59.852.6 16.2 0.98 Alloy 6 444.5 899.5 61.45 62.74 434.4 875.6 55.5 55.212.0 0.89

Hydrogen induced cracking (HIC) and Sulfide Stress Cracking (SSC) testswere completed on Alloy 4 according to NACE TM0284-2016, and NACETM0177-2016—Method A, respectively. The applied stress during the SSCtest was 85% of the actual yield stress to simulate a higher hydrogenpressure environment. No cracks were present after each test. Somepitting corrosion was observed in the SSC test.

TABLE 4 Sulfide Stress Cracking (SSC) Testing Applied Temper- TestSpecimen % Stress ature Period Initial Final ID AYS (psi) (° C.) (hours)pH pH Result 865 85 54,400 24 720 2.63 3.10 Pass* Specification: NACETM0177-2016 - Method A Test Environment: NACE TM0177 Solution A, pH =2.7, 100 mol. % H = S Test Specimen: Type: Standard OrientationLongitudinal Finishing Process: Ground Loading Method: Proof Ring*Indications were noted on gauge length. The cross section was examined,and no SSC was observed.

The microstructures of the electrochemically charged tensile fractureexhibited ductile fracture features in both the uncharged and hydrogencharged samples throughout the entire fractured surface. Microstructuresof the fractured surface of Alloy 6 before and after electrochemicallyhydrogen charging were taken at the edge (FIGS. 9 and 10 ), quarter(FIGS. 11 and 12 ) and center (FIGS. 13 and 14 ) of the round tensilesample.

The low-Ni austenitic grade design proposed in this study with carbonlevels between 0.18 weight percent and 0.5 weight percent and manganeselevels between 16.5 to 24.5 weight percent, along with the Al, Cr, Cu,Ni, and Si levels proved to be resistant to hydrogen embrittlement,producing results less than 20 weight percent RRA afterelectrochemically charging for ingress of nascent hydrogen atoms. Basedon this study and literature review, materials with carbon levelsbetween 0.18 to 0.6 weight percent, manganese levels between 16 to 30weight percent, chromium levels between 2.0 and 3.5 weight percent,copper levels between 0.6 to 2 weight percent, nickel levels greaterthan 0.9 weight percent with an aim of 1.3 weight percent for costreduction purposes, silicon levels between 2.0 weight percent and 4.0weight percent, and aluminum levels between 0.04 weight percent to 2weight percent show to be suitable affordable low-Ni austenitesubstitutes to 316L stainless steel in resisting hydrogen degradation.In this study, the grade with both low carbon and low Mn was moresusceptible to hydrogen embrittlement. Titanium may be used to increasemechanical properties, but titanium may also increase the material'ssusceptibility to hydrogen embrittlement.

As used herein, “including,” “containing” and like terms are understoodin the context of this application to be synonymous with “comprising”and are therefore open-ended and do not exclude the presence ofadditional undescribed or unrecited elements, materials, phases ormethod steps. As used herein, “consisting of” is understood in thecontext of this application to exclude the presence of any unspecifiedelement, material, phase or method step. As used herein, “consistingessentially of” is understood in the context of this application toinclude the specified elements, materials, phases, or method steps,where applicable, and to also include any unspecified elements,materials, phases, or method steps that do not materially affect thebasic or novel characteristics of the invention.

Notwithstanding that the numerical ranges and parameters setting forththe broad scope of the invention are approximations, the numericalvalues set forth in the specific examples are reported as precisely aspossible. Any numerical value, however, inherently contains certainerrors necessarily resulting from the standard variation found in theirrespective testing measurements.

Also, it should be understood that any numerical range recited herein isintended to include all sub-ranges subsumed therein. For example, arange of “1 to 10” is intended to include all sub-ranges between (andincluding) the recited minimum value of 1 and the recited maximum valueof 10, that is, having a minimum value equal to or greater than 1 and amaximum value of equal to or less than 10.

In this application, the use of the singular includes the plural andplural encompasses singular, unless specifically stated otherwise. Inaddition, in this application, the use of “or” means “and/or” unlessspecifically stated otherwise, even though “and/or” may be explicitlyused in certain instances. In this application and the appended claims,the articles “a,” “an,” and “the” include plural referents unlessexpressly and unequivocally limited to one referent.

Whereas particular embodiments of this invention have been describedabove for purposes of illustration, it will be evident to those skilledin the art that numerous variations of the details of the presentinvention may be made without departing from the invention as defined inthe appended claims.

What is claimed is:
 1. A hydrogen degradation resistant steel alloycomprising from 15 to 30 weight percent Mn, from 0.15 to 1 weightpercent C, and from 0.05 to 3 weight percent Al, wherein the steel alloyhas a microstructure comprising at least 99 percent volume austenite,and possesses a relative reduction in area of no more than 20 percent.2. The steel alloy of claim 1, wherein the C is greater than 0.2 weightpercent.
 3. The steel alloy of claim 1, wherein the Mn is greater than18 weight percent.
 4. The steel alloy of claim 1, wherein when the Mn isless than 18 weight percent, the C is greater than 0.2 weight percent.5. The steel alloy of claim 1, wherein when the C is less than 0.3weight percent, the Mn is greater than 18 weight percent.
 6. The steelalloy of claim 1, wherein the Mn comprises from 18 to 25 weight percent,and the C comprises from 0.3 to 1 weight percent.
 7. The steel alloy ofclaim 1, wherein the Mn comprises from 20 to 24 weight percent, and theC comprises from 0.4 to 0.6 weight percent.
 8. The steel alloy of claim1, further comprising from 0.8 to 2.5 weight percent Ni.
 9. The steelalloy of claim 1, further comprising at least 0.2 weight percent Cu. 10.The steel alloy of claim 1, further comprising from 0.8 to 2.5 weightpercent Ni, and from 0.2 to 2 weight percent Cu.
 11. The steel alloy ofclaim 10, further comprising at least 0.5 weight percent Si.
 12. Thesteel alloy of claim 10, further comprising at least 1 weight percentCr.
 13. The steel alloy of claim 10, further comprising from 0.5 to 4weight percent Si, and from 1 to 3.5 weight percent Cr.
 14. The steelalloy of claim 1, further comprising at least 0.02 weight percent Ti.15. The steel alloy of claim 1, wherein the Mn comprises from 20 to 24weight percent, and the C comprises from 0.3 to 0.6 weight percent. 16.The steel alloy of claim 15, further comprising from 0.8 to 2.5 weightpercent Ni, and from 0.2 to 2 weight percent Cu.
 17. The steel alloy ofclaim 16, further comprising from 0.5 to 4 weight percent Si, and from 1to 3.5 weight percent Cr.
 18. The steel alloy of claim 17, wherein theAl comprises from 1.4 to 1.8 weight percent, the Ni comprises from 1.2to 1.5 weight percent, the Cu comprises from 0.6 to 1.2 weight percent,the Si comprises from 2 to 3.2 weight percent, and the Cr comprises from2 to 3.2 weight percent.
 19. The steel alloy of claim 18, furthercomprising from 0.08 to 0.2 weight percent Ti.
 20. The steel alloy ofclaim 1, wherein the microstructure comprises at least 99.5 volumepercent austenite.
 21. The steel alloy of claim 1, wherein the relativereduction in area is less than 15 percent.
 22. The steel alloy of claim1, wherein the steel alloy possesses an ultimate tensile strength ofgreater than 700 MPa, and a total elongation of greater than 50 percent.23. A method of producing the steel alloy of claim 1, comprising hotrolling the steel alloy to form a slab and cooling the slab.
 24. Themethod of claim 23, further comprising subjecting the steel alloy slabto electrochemical charging to generate nascent atomic hydrogen.